(a) Field of the Invention
The present invention relates to a method for preparing lithium manganese complex oxide, Li1+xMn2−xO4 (0×0.12), with a spinel structure used for a cathode active material of a lithium or lithium ion secondary battery.
(b) Description of the Related Art
LiCoO2 compound is most commonly used for a cathode active material of a lithium or lithium ion secondary battery having an electric potential of 4 V (voltage). However, many studies for other active materials have been conducted recently, because LiCoO2 compound is expensive and disadvantageous in terms of stability. A lithium manganese complex oxide, Li1+xMn2−xO4 (0≦x≦0.12), with a spinel structure, is one of the most actively studied materials, because it is inexpensive and stable during its usage, and it has an environmental affinity.
The conventional method for synthesizing lithium manganese complex oxide with a spinel structure comprises mixing a manganese compound and a lithium compound so as to adjust their chemical compositions, and heat-treating the mixture at a high temperature. U.S. Pat. No. 5,718,877 discloses a heat-treatment process for obtaining chemically uniform spinel, and R. J. Gummow et al., Solid State Ionics, 69, 59(1994) state that a compound with a spinel structure is not limited to the stoichiometry of LiMn2O4, and a spinel structure forms even if x in Li1+xMn2−xO4 varies from 0 to 0.33, and, as x increases, the valence of Mn reaches 4 and the crystalline structure becomes stabilized.
After it was reported that the deterioration of electrochemical characteristics of lithium manganese complex compound with a spinel structure is promoted by nonuniformity of spinel composition, various studies for synthesizing more chemically uniform spinel using a liquid phase method have been attempted. However, with most liquid phase methods, a spinel compound having a particle size of several μm or less is obtained. Such micro particles are difficult to use as a cathode active material of a battery, because they have poor particle flow characteristics, charge density, tap density, and wettability for solvent, and thus they have many problems in the electrode preparation process, even if each particle has excellent electrochemical properties.
Recently, it was found that defects in raw materials, particularly electrolytic manganese dioxide (EMD), promote nonuniformity of the composition or local defects of structure when synthesizing spinel, and deteriorate the electrochemical properties of spinel. Inside EMD, a raw material of manganese, a variety of defects introduced during the process of synthesis (impurities, absorbed water, crystalline water, hydrogen ions, and other ions such as SO42−, Cl, NH4+, etc.) exist. Such impurities form a stable mesophase that exists as impurities during the heat-treatment process for obtaining the spinel compound of Li1+xMn2−xO4. In addition, such defects may deteriorate performance when used as a cathode active material of a lithium or lithium ion secondary battery, because the synthesized compound has internal lattice defects, even if it has a spinel structure. Specifically, lithium diffuses into a lattice of a spinel compound when charging/discharging, and the partial lattice defects inhibit the movement of lithium in the process of insertion and deintercalation, as well as react with lithium to decrease the amount of movable lithium, and consequently decreases the capacity of the battery.
There have been many attempts at removing the above-mentioned defects. The most representative method is acid treating or base treating the raw material powder in a strong acid or a strong base. The acid treatment method is used to remove impurity metal ions existing in the raw material, and the base treatment method is used to substitute hydrogen ions in the raw material with lithium. Since these methods are used with aqueous solutions, they involve the risk of moisture infiltration into the raw material and the absorbed water may be incorporated therein. In addition, since particles strongly aggregate after they are dried of the aqueous solution, the above methods involve a pulverizing process, during which process the impurities may be incorporated again, and they have the inconvenience that an acid treatment cannot be conducted simultaneously with a base treatment to remove the impurities.
Another inherent problem is the shape of the secondary aggregate of the manganese raw material. When the mixture of manganese dioxide and lithium compound is heat-treated, lithium infiltrates the manganese compound and a reaction proceeds, and the shape of the formed spinel nearly maintains the shape of the particle of manganese raw material. Therefore, in order to control the shape of the spinel particle, the shape of the particle of manganese dioxide should be controlled.